Carbon dioxide gas absorbent and carbon dioxide gas separating apparatus

ABSTRACT

There is disclosed a carbon dioxide gas absorbent comprising lithium silicate, 0.5 mol % to 4.9 mol % of alkali carbonate per mole of the lithium silicate, and at least one element selected from the group consisting of aluminum, magnesium, calcium, iron, titanium and carbon.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based upon and claims the benefit of priorityfrom the prior Japanese Patent Application No. 2001-299131, filed Sep.28, 2001, the entire contents of which are incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to a carbon dioxide gas absorbent and acarbon dioxide gas separating apparatus.

[0004] 2. Description of the Related Art

[0005] If it is desired to perform the separation and recovery of carbondioxide gas from the exhaust gas that has been discharged from acombustion apparatus such as an engine where a fuel containing mainly ofhydrocarbons is combusted, it is effective to perform the separation andrecovery of carbon dioxide gas at a location which is close to thecombustion chamber and therefore is high in the concentration of carbondioxide gas. Since this location which is close to the combustionchamber is relatively high in temperature, unless the exhaust gasdischarged from the combustion chamber is cooled by a heat exchanger,etc., the separation and recovery of carbon dioxide gas is inevitablyrequired to be performed under an environment of about 300° C. or more.

[0006] As for the method of separating and recovering carbon dioxidegas, there have been conventionally proposed various methods such as amethod where cellulose acetate is employed, a method where an alkanolamine type solvent is employed, and a chemical absorption method wherecarbon dioxide is absorbed by using a solution. In any of theseconventional carbon dioxide gas separation/recovery methods, theabsorption of carbon dioxide gas is performed at a temperature of about200° C. or less. Because of this, the temperature of the exhaust gascontaining carbon dioxide gas on the occasion of introducing it into anabsorption system is required to be controlled to be about 200° C. orless. Therefore, the exhaust gas having a high concentration of carbondioxide gas and existing in the vicinity of the combustion chamber isrequired to be cooled to a temperature of about 200° C. or less by aheat exchanger, etc. before the carbon dioxide gas is to be separatedand recovered. As a result, there is a problem that the energyconsumption for the separation and recovery of carbon dioxide isinevitably increased.

[0007] On the other hand, Jpn. Pat. Appln. KOKAI Publication No. 9-99214discloses a carbon dioxide gas absorbent comprising lithium zirconate.Further, Jpn. Pat. Appln. KOKAI Publication No. 2000-262,890 and Jpn.Pat. Appln. KOKAI Publication No. 2001-170,480 discloses a carbondioxide gas absorbent comprising lithium silicate. Lithium zirconate andlithium silicate are capable of absorbing carbon dioxide gas attemperatures exceeding about 500° C. Further, when lithium zirconate andlithium silicate are heated to a temperature of 600° C. or more, carbondioxide is desorbed therefrom. Moreover, when at least one kind ofalkali carbonate selected from the group consisting of lithiumcarbonate, potassium carbonate and sodium carbonate is added to lithiumzirconate and lithium silicate, the carbon dioxide gas-absorbingproperty of the carbon dioxide gas absorbent can be improved, so that itbecomes possible to effectively absorb carbon dioxide gas even if carbondioxide is of low concentration.

[0008] However, these conventional carbon dioxide gas absorbentsincorporating alkali carbonates are accompanied with problems that theserviceable life thereof is relatively short, and that if the separationand recovery of carbon dioxide gas are to be performed using theseconventional carbon dioxide gas absorbents, it requires a relativelyhigh temperature for releasing carbon dioxide gas from the carbondioxide gas absorbent.

BRIEF SUMMARY OF THE INVENTION

[0009] Therefore, an object of the present invention is to provide acarbon dioxide gas absorbent which is long in serviceable life.

[0010] A further object of the present invention is to provide a carbondioxide gas separating apparatus which is capable of effectivelyperforming the separation and recovery of carbon dioxide gas.

[0011] According to one aspect of the present invention, there isprovided a carbon dioxide gas absorbent comprising:

[0012] lithium silicate;

[0013] 0.5 mol % to 4.9 mol % of alkali carbonate per mole of thelithium silicate; and

[0014] at least one element selected from the group consisting ofaluminum, magnesium, calcium, iron, titanium and carbon.

[0015] According to another aspect of the present invention, there isprovided a carbon dioxide gas separating apparatus comprising:

[0016] a reaction chamber having a carbon dioxide gas inlet and aproduct gas outlet;

[0017] a carbon dioxide gas absorbent placed in the reaction chamber;the carbon dioxide gas absorbent comprising lithium silicate; 0.5 mol %to 4.9 mol % of alkali carbonate per mole of the lithium silicate; andat least one element selected from the group consisting of aluminum,magnesium, calcium, iron, titanium and carbon; and

[0018] a heater heating the reaction chamber and disposed around thereaction chamber.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0019]FIG. 1 schematically illustrates a carbon dioxide gas separatingapparatus according to one embodiment of the present invention; and

[0020]FIG. 2 schematically illustrates a carbon dioxide gas separatingapparatus according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0021] When an alkali carbonate is incorporated, according to theconventional procedure, into lithium silicate employed as a carbondioxide gas absorbent, the following problems occur. More specifically,a relatively long time and a high temperature are required for effectingthe carbon dioxide gas-releasing reaction. If the carbon dioxidegas-releasing reaction is continued to proceed at a relatively hightemperature, the grains of the carbon dioxide gas absorbent becomelarger, which decreases the porosity of the carbon dioxide gasabsorbent, so that the carbon dioxide absorption/desorptioncharacteristics of the carbon dioxide gas absorbent is deteriorated,thus shortening the serviceable life thereof.

[0022] With a view to investigate the cause of this phenomenon, thepresent inventors have analyzed the interface of grains of the carbondioxide gas absorbent after permitting carbon dioxide gas to be absorbedtherein and found the following facts. Namely, there was recognized, onthe interface of the grains of the carbon dioxide gas absorbent, theformation of a film comprising a reaction product formed through areaction between impurities included in the carbon dioxide gas absorbentand the alkali carbonate which was incorporated into the carbon dioxidegas absorbent for promoting the absorption of carbon dioxide. As carbondioxide gas is absorbed by the carbon dioxide gas absorbent, solidlithium carbonate is formed on the surface of the carbon dioxide gasabsorbent. In this case, alkali carbonate functions to liquefy thissolid lithium carbonate formed as mentioned above, thereby enhancing thediffusing rate of carbon dioxide gas onto the surface of the carbondioxide gas absorbent. However, this liquefied lithium carbonateconcurrently reacts with the aforementioned impurities to form a film.

[0023] When this film is heated higher than the melting point thereof,it can be liquefied, but when this film is kept lower than the meltingpoint thereof, the carbon dioxide gas-releasing reaction in the carbondioxide gas absorbent is prevented. Therefore, in order to enable theabsorbed carbon dioxide gas to be completely desorbed, the carbondioxide gas absorbent is required to be heated up to a temperature whichenables the film to be liquefied. Since a relatively high temperature isrequired for enabling the carbon dioxide gas-releasing reaction to takeplace as described above, the grains of the carbon dioxide gas absorbentgrow, thereby degrading the carbon dioxide gas absorbent and henceshorten the serviceable life of the carbon dioxide gas absorbent.

[0024] As for the aforementioned impurities that may be included in thecarbon dioxide gas absorbent and forming the film together with alkalicarbonate, they include at least one kind of material selected from thegroup consisting of aluminum, magnesium, calcium, iron, titanium andcarbon. These impurities originate from the raw materials of the carbondioxide gas absorbent, i.e. silicon dioxide and lithium carbonate, orfrom the course of processing. These impurities are unavoidably includedin the carbon dioxide gas absorbent.

[0025] In order to enable alkali carbonate to function as an acceleratorfor the carbon dioxide gas-absorbing reaction, the content of alkalicarbonate in the lithium silicate is required to be at least about 0.5mol %. Further, it has been found by the present inventors that in orderto prevent the carbon dioxide gas absorbent from being deteriorated incarbon dioxide gas-releasing property thereof even if the carbon dioxidegas absorbent contains an unavoidable impurity, the content of alkalicarbonate in the lithium silicate is required to be limited to a certainlevel. Namely, when the content of alkali carbonate in the lithiumsilicate exceeds about 4.9 mol %, the melting point of the film to beformed through the reaction between an impurity and an alkali carbonateis sharply raised to as high as 850° C. or so. As a result, the carbondioxide gas-releasing property of the carbon dioxide gas absorbent isdeteriorated. Therefore, in the carbon dioxide gas absorbent accordingto the embodiments of the present invention, the content of alkalicarbonate should preferably be confined within the range of about 0.5mol % to about 4.9 mol %. In the carbon dioxide gas absorbent accordingto the embodiments of the present invention, it may contain, as animpurity, at least one kind of element selected from the groupconsisting of aluminum, magnesium, calcium, iron, titanium and carbon.

[0026] When the content of an alkali carbonate is confined within therange of about 0.5 mol % to about 4.9 mol %, a carbon dioxide gasabsorbent having a long serviceable life can be obtained. The carbondioxide gas absorbent confined in this manner is excellent in carbondioxide gas-absorbing property in a temperature range which is as highas around 500° C. and also in carbon dioxide gas-releasing property in atemperature range exceeding over about 700° C. If it is desired toenable the carbon dioxide gas-releasing reaction to be effectivelyexecuted in the carbon dioxide gas absorbent, the content of the alkalicarbonate should preferably be confined within the range of about 1 mol% to about 3 mol %. 5 The lithium silicate to be employed in the carbondioxide gas absorbent is capable of efficiently reacting with carbondioxide gas at temperatures of around 500° C. to generate silicondioxide and lithium carbonate, thereby absorbing carbon dioxide gas.Since this reaction is reversible, silicon dioxide and lithium carbonatereact with each other at a temperature of about 700° C. to generatelithium silicate and carbon dioxide gas. Since lithium silicate iscapable of selectively absorbing carbon dioxide gas and also capable ofdesorbing the carbon dioxide gas that has been once absorbed therein, itbecomes possible to recycle the carbon dioxide gas that has beenseparated and recovered by the lithium silicate. Further, since thiscarbon dioxide gas absorbent is capable of reversibly absorbing anddesorbing carbon dioxide gas, it can be repeatedly used.

[0027] Moreover, the carbon dioxide gas absorbent according to theembodiments of the present invention is capable of absorbing carbondioxide gas even at room temperature. In this case also, it is possibleto separate the carbon dioxide gas that has been once absorbed by thecarbon dioxide gas absorbent from lithium silicate by heating the carbondioxide gas absorbent up to a temperature of about 700° C. as mentionedabove.

[0028] As for the lithium silicate, it is possible to employ a compoundrepresented by a formula: Li_(x)Si_(y)O_(z) (wherein x+4y−2z=0). Forexample, it is possible to employ at least one kind of compound selectedfrom the group consisting of lithium orthosilicate (Li₄SiO₄), lithiummetasilicate (Li₂SiO₃), Li₆Si₂O₇ and Li₈SiO₆. These various kinds oflithium silicate may be employed singly or in a combination of two ormore kinds. Among them, due to the features that the temperature thatmakes a boundary point between the absorption of carbon dioxide gas andthe desorption of carbon dioxide gas relatively high and hence theseparation of carbon dioxide gas can be executed at relatively hightemperature, the employment of lithium orthosilicate is especiallypreferable. Incidentally, these various kinds of lithium silicate maydiffer more or less in composition from the stoichiometric ratio in thechemical formula.

[0029] As for the alkali carbonate, it is possible to employ at leastone selected from the group consisting of potassium carbonate, sodiumcarbonate and lithium carbonate. These various kinds of alkali carbonatemay be employed singly or in a combination of two or more kinds. Amongthem, because of the features that the carbon dioxide gas absorbingproperty is relatively high, the employment of potassium carbonate isespecially preferable. Incidentally, these various kinds of alkalicarbonate may differ more or less in composition from the stoichiometricratio in the chemical formula.

[0030] The carbon dioxide gas absorbent according to the embodiments ofthe present invention can be employed in the form of powder or in theform of a porous body. A mixture comprising powdery one and porous onemay be used. The porous carbon dioxide gas absorbent can be formed bycompressing the powder of the carbon dioxide gas absorbent to such anextent that enables carbon dioxide gas to pass through the interior ofthe resultant compressed body. If the carbon dioxide gas absorbent is tobe employed in the form of powder, the packing quantity of the carbondioxide gas absorbent into the interior of the chamber can be increased,thus preferably increasing the absorption quantity of carbon dioxidegas. On the other hand, if the carbon dioxide gas absorbent is to beemployed in the form of a porous body, the clogging of the carbondioxide gas absorbent by an exhaust gas flow of high flow rate can beminimized, thus preferably minimizing the pressure loss of the gas flow.Irrespective of whether the carbon dioxide gas absorbent is employed inthe form of powder or the carbon dioxide gas absorbent is employed inthe form of a porous body by molding the powder thereof, it is requiredto suppress the aggregation of the powder of the carbon dioxide gasabsorbent and also to increase the contacting surface thereof with anexhaust gas. Because of this, the average particle diameter of thepowder should preferably be confined within the range of about 0.5 μm toabout 50 μm. The average particle diameter of the powder can be measuredby a laser diffraction method.

[0031] On the occasion of forming a porous body from the powder thereof,it is required to form the porous body in such a manner that theresultant porous body is capable of suppressing the pressure loss of gasflow and capable of retaining the mechanical strength to a certainextent. Therefore, it is preferable that the porosity of the porous bodyis confined within the range of about 30% to about 60%. Where the carbondioxide gas absorbent is to be employed in the form of a porous body,alkali carbonate can be retained by the fine pores or fine voids of theporous body, so that the external dimensions of the porous body needhardly to be changed.

[0032] The carbon dioxide gas absorbent having such a porous structureas described above can be manufactured by the following method.

[0033] First of all, predetermined quantities of silicon dioxide andlithium carbonate are weighed-out, and are then mixed together for about0.1 to 1.0 hours by using an agate mortar. The resultant mixed powder isplaced in an alumina crucible and heat-treated for about 0.5 to 20 hoursin an air atmosphere in a box type electric furnace. As a result, a rawlithium silicate powder can be obtained. Then, a predetermined quantityof potassium carbonate is added as an alkali carbonate to this rawlithium silicate powder and dry-mixed to obtain a mixture. Thereafter, apredetermined quantity of the mixture comprising lithium silicate andpotassium carbonate is weighed and introduced into a mold. The mixtureis then press-molded to form a molded body having a porosity of about40%, thereby obtaining a porous body of the carbon dioxide gasabsorbent.

[0034] The carbon dioxide gas absorbent according to the embodiments ofthe present invention can be used in a carbon dioxide gas separatingapparatus as explained below.

[0035]FIG. 1 is a cross-sectional view schematically illustrating acarbon dioxide gas separating apparatus according to one embodiment ofthe present invention.

[0036] A first absorption pipe 1 ₁ and a second absorption pipe 1 ₂ arerespectively formed of a double structure, each comprising an inner pipe2 ₁ and an outer pipe 3 ₁ and an inner pipe 2 ₂ and an outer pipe 3 ₂The inner spaces of the inner pipes 2 ₁ and 2 ₂ constitute a firstreaction chamber 21 ₁ and a second reaction chamber 21 ₂, respectively,while the spaces formed between the inner pipes 2 ₁ and 2 ₂ and theouter pipes 3 ₁ and 3 ₂ constitute a first heater (heating means) 20 ₁and a second heater (heating means) 20 ₂, respectively. The first andsecond reaction chambers 21 ₁ and 21 ₂ are heated by the first andsecond heaters 20 ₁ and 20 ₂, respectively.

[0037] The first and second reaction chambers 21 ₁ and 21 ₂ are filledwith a carbon dioxide gas absorbent as represented by 4 ₁ and 4 ₂,respectively. A first and second carbon dioxide gas-containing gassupply branch pipes 6 ₁ and 6 ₂, both branched from a carbon dioxidegas-containing gas supply pipe 5, are respectively coupled with an upperend of each of the first and second reaction chambers 21 ₁ and 21 ₂. Thefirst and second carbon dioxide gas-containing gas supply branch pipes 6₁ and 6 ₂ are provided with a first valve 7 ₁ and a second valve 7 ₂respectively.

[0038] A first and second recovering gas supply branch pipes 9 ₁ and 9 ₂both branched from a carbon dioxide gas-recovering gas supply pipe 8,are coupled with an upper end of each of the first and second reactionchambers 21 ₁ and 21 ₂, respectively. The first and second recoveringgas supply branch pipes 9 ₁ and 9 ₂ are provided with a third valve 7 ₃and a fourth valve 7 ₄, respectively.

[0039] One end of each of a first and second gas discharge branch pipes10 ₁ and 10 ₂ is coupled with a bottom portion of each of the first andsecond reaction chambers 21 ₁ and 21 ₂, while the other end of each ofthe first and second gas discharge branch pipes 10 ₁ and 10 ₂ is coupledwith a processed gas discharge pipe 11 which is provided with a fifthvalve 7 ₅. One end of each of a first and second recovering gasdischarge branch pipes 12 ₁ and 12 ₂ is coupled with a bottom portion ofeach of the first and second reaction chambers 21 ₁ and 21 ₂, while theother end of each of the first and second recovering gas dischargebranch pipes 12 ₁ and 12 ₂ is coupled with a recovering gas dischargepipe 13 which is provided with a sixth valve 7 ₆.

[0040] A combustor 14 for combusting a fuel gas is disposed next to thefirst absorption pipe 1 ₁. One end of a combustion gas supply pipe 15 iscoupled with the combustor 14, while the other end of the combustion gassupply pipe 15 is diverged into a first and second combustion gas supplybranch pipes 16 ₁ and 16 ₂. The first and second combustion gas supplybranch pipes 16 ₁ and 16 ₂ are coupled with a lower sidewall of each ofthe first and second heaters 20 ₁ and 20 ₂, respectively, and providedwith a seventh and eighth valves 7 ₇ and 7 ₈ respectively.

[0041] First and second exhaust pipes 17 ₁ and 17 ₂ are communicatedwith the first and second heaters 20 ₁ and 20 ₂, respectively. As a fuelgas is introduced into the combustor 14 and combusted therein, thecombustion gas generated by the combustion of the fuel gas istransferred, via the combustion gas supply pipe 15 and the first andsecond combustion gas supply branch pipes 16 ₁ and 16 ₂, to the firstand second heaters 20 ₁ and 20 ₂, respectively. The combustion gas ispermitted to flow through the inner space of the first and secondheaters 20 ₁ and 20 ₂ and is discharged from the first and secondexhaust pipes 17 ₁ and 17 ₂. During the time when the combustion gaspasses through the aforementioned inner space, the carbon dioxide gasabsorbents 4 ₁ and 4 ₂ loaded inside the first and second reactionchambers 21 ₁ and 21 ₂ are heated.

[0042] The number of moles per unit time of the gas flowing through thefirst and second reaction chambers 21 ₁ and 21 ₂ should preferably beset to about four to about 50 times larger than the number of moles ofthe carbon dioxide gas absorbents 4 ₁ and 4 ₂ loaded therein. When thenumber of moles per unit time of the gas flow exceeds over 50 times, itbecomes difficult to effectively perform the absorption of carbondioxide gas in view of the capacity utilization of the first and secondreaction chambers 21 ₁ and 21 ₂. On the other hand, when the number ofmoles per unit time of the gas flow is less than about 4 times, thegeneration of heat due to the absorption reaction becomes excessive toraise the temperature of the gas flow, thus possibly obstructing theabsorption reaction itself. In view of both of the utilizationefficiency of the capacity of the reaction chamber and the rapidprogress of the absorption reaction, it is more preferable to confinethe number of moles per unit time of the gas flowing to about 8 to about30 times larger than the number of moles of the carbon dioxide gasabsorbents 4 ₁ and 4 ₂ loaded therein.

[0043] It is possible, through the employment of the first and secondreaction chambers 21 ₁ and 21 ₂ filled with the carbon dioxide gasabsorbents 4 ₁ and 4 ₂ to continuously perform the absorption andrecovery of carbon dioxide gas by alternately executing the carbondioxide gas absorption reaction and the carbon dioxide gas desorptionreaction by following the procedures (1-1) and (1-2) as explained below.

[0044] (1-1)The Procedure of Carbon Dioxide Gas Absorption at the FirstAbsorption Pipe 1 ₁:

[0045] First of all, the first valve 7 ₁ mounted on the first carbondioxide gas-containing gas supply branch pipe 6 ₁ which is coupled withthe first reaction chamber 21 ₁ and the fifth valve 7 ₅ mounted on theprocessed gas discharge pipe 11 are opened while closing all of theother valves, i.e. valves 7 ₂, 7 ₃, 7 ₄, 7 ₆, 7 ₇ and 7 ₈ A carbondioxide gas-containing gas is then permitted to flow from the carbondioxide gas-containing gas supply pipe 5, via the first carbon dioxidegas-containing gas supply branch pipe 6 ₁, to the first reaction chamber21 ₁. In this case, as mentioned above, the number of moles per unittime of the gas flowing through the first reaction chamber 21 ₁ is setto about 4 to about 50 times larger than the number of moles of thelithium silicate being filled in the first reaction chamber 21 ₁. As aresult, it is possible to enable the carbon dioxide gas in the carbondioxide gas-containing gas to be rapidly absorbed and retained by thecarbon dioxide gas absorbent 4 ₁. The gas where the concentration ofcarbon dioxide gas is reduced in this manner is permitted to passthrough the first gas discharge branch pipe 10 ₁ and the processed gasdischarge pipe 11 so as to be discharged out of the system.

[0046] The absorption of carbon dioxide gas at the second absorptionpipe 1 ₂ can be performed in the same manner as described above.

[0047] (1-2)The Procedure of the Recovery of Carbon Dioxide Gas at theSecond Absorption Pipe 1 ₂:

[0048] During the time when the procedure of carbon dioxide gasabsorption is being performed at the first absorption pipe 1 ₁ asexplained in the above item (1-1), the fourth valve 7 ₄ mounted on thesecond recovering gas supply branch pipe 9 ₂ which is coupled with thesecond absorption pipe 1 ₂ and the sixth valve 7 ₆ mounted on therecovering gas discharge pipe 13, and the eighth valve 7 ₈ mounted onthe second combustion gas supply branch pipe 16 ₂ are opened.

[0049] Subsequently, the combustion gas from the combustor 14 is passed,via the combustion gas supply pipe 15 and the second combustion gassupply branch pipe 16 ₂, to the second heater 20 ₂. As a result, thecarbon dioxide gas absorbent 4 ₂ loaded in the second reaction chamber21 ₂ is heated up to about 800° C. or more, and at the same time, adesired recovering gas is fed from the carbon dioxide gas-recovering gassupply pipe 8, via the second recovering gas supply branch pipe 9 ₂, tothe second reaction chamber 21 ₂. On this occasion, the carbon dioxidegas that has already been absorbed by the carbon dioxide gas absorbent 4₂ is rapidly released due to the generation of carbon dioxide desorptionreaction, and the gas flow containing a high concentration of carbondioxide gas is recovered by the second recovering gas discharge branchpipe 12 ₂ and the recovering gas discharge pipe 13.

[0050] The recovery of carbon dioxide gas from the first absorption pipe1 ₁ can be performed in the same manner as explained above.

[0051] As described above, on the occasion of performing the carbondioxide gas absorption procedure at the first absorption pipe 1 ₁, theprocedure of recovering carbon dioxide gas from the second absorptionpipe 1 ₂ can be performed concurrent therewith. Further, on the occasionof the procedure to recover carbon dioxide gas from the first absorptionpipe 1 ₁, the procedure of absorbing carbon dioxide gas at the secondabsorption pipe 1 ₂ can be performed concurrent therewith. It ispossible, by alternately repeating these procedures, to realize acontinuous separation of carbon dioxide gas.

[0052] Any of the inner pipes 2 ₁ and 2 ₂, the outer pipes 3 ₁ and 3 ₂the first and second carbon dioxide gas-containing gas supply branchpipes 6 ₁ and 6 ₂, the first and second recovering gas supply branchpipes 9 ₁ and 9 ₂ the first and second gas discharge branch pipes 10 ₁and 10 ₂, and the first and second recovering gas discharge branch pipes12 ₁ and 12 ₂ can be constituted by any desired material. For example,high-density alumina, and metals such as nickel and iron can be employedfor constituting these members. Further, in order to effectively isolatecarbon dioxide gas to be generated in the first and second reactionchambers 21 ₁ and 21 ₂, it is preferable to increase the capacity of thefirst and second heaters 20 ₁ and 20 ₂. Further, if it is desired toprolong the contact time between the fuel gas and the carbon dioxide gasabsorbents 4 ₁ and 4 ₂ the employment of a tubular configurationextended in the direction of the gas flow would be desirable for shapeof the first and second chambers 21 ₁ and 21 ₂.

[0053] Additionally, depending on the reaction temperature of the rawgas, a temperature controller such as a heater may be installed insideor outside the reaction chamber so as to make it possible, if desired,to set the temperature of the interior of the reaction chamber to adesired temperature.

[0054] As explained above, it is possible, according to the embodimentof the present invention, to provide a carbon dioxide gas separatingapparatus which is simple in structure, cheap in manufacturing cost, andcapable of continuously performing the separation and recovery of carbondioxide gas. Furthermore, since a carbon dioxide gas absorbent having along serviceable life can be employed in the embodiment, the separationand recovery of carbon dioxide gas can be effectively performed.

[0055] Next, specific examples of the present invention will beexplained in detail.

EXAMPLE I-1

[0056] Lithium carbonate powder having an average particle diameter ofabout 1 μm, and silicon dioxide powder having an average particlediameter of about 0.8 μm were weighed so as to make the molar ratiothereof about 2:1. Then, these raw materials were dry-mixed in an agatemortar for about 10 minutes to obtain a mixture, which was thenheat-treated at a temperature of about 1,000° C. for about 8 hours in anair atmosphere in a box type electric furnace to obtain lithiumorthosilicate powder.

[0057] Then, potassium carbonate having an average particle diameter ofabout 1 μm was added to this lithium orthosilicate powder at a ratio ofabout 2 mol % and dry-mixed in an agate mortar. Thereafter, the mixturecomprising lithium silicate and potassium carbonate was introduced intoa mold having a diameter of about 12 mm. The mixture is thenpress-molded to form a porous carbon dioxide gas absorbent having aporosity of about 40%.

[0058] It was found that this porous carbon dioxide gas absorbentcontained as an impurity about 550 ppm of aluminum, about 220 ppm ofmagnesium, about 500 ppm of calcium, about 50 ppm of iron, and about 50ppm of titanium.

EXAMPLE I-2 to I-6, and Comparative EXAMPLES I-1 and I-2

[0059] Various kinds of porous carbon dioxide gas absorbents weremanufactured by repeating the same procedures and using the materials asdescribed in Example I-1, except that the kind and the content of alkalicarbonate was varied as shown in the following Table 1. Since thesecarbon dioxide gas absorbents were prepared by the same process usingthe same materials as described in Example I-1, almost the quantity andkinds of impurities were included therein as an impurity. TABLE 1 Alkalicarbonates Content Kinds (mol %) Example I-1 K₂CO₃ 2 Example I-2 K₂CO₃0.5 Example I-3 K₂CO₃ 1 Example I-4 K₂CO₃ 3 Example I-5 K₂CO₃ 4.9Example I-6 Na₂CO₃ 2 Comparative K₂CO₃ 5 Example I-1 Comparative K₂CO₃0.3 Example I-2

[0060] The carbon dioxide gas absorbents obtained in Examples I-1 toI-6, and Comparative Examples I-1 and I-2 were respectively housedinside the box type electric furnace. Then, this electric furnace wasmaintained at a temperature of about 500° C. while permitting a mixedgas comprising about 20 vol % of carbon dioxide gas and about 80 vol %of nitrogen gas to flow therethrough. Then, the time where the weight ofthe carbon dioxide gas absorbent that had absorbed carbon dioxide gaswas increased to about 130 wt % of the initial weight was measured toevaluate the carbon dioxide gas-absorption performance of these carbondioxide gas absorbents.

[0061] Further, after having carbon dioxide gas absorbed therein underthe aforementioned conditions, these carbon dioxide gas absorbents wereonce cooled to room temperature to measure the weight thereof.Thereafter, these carbon dioxide gas absorbents were kept at atemperature of about 800° C. under a gaseous condition where carbondioxide gas was set about 100 vol % to measure the reduction of theweight thereof, thus evaluating the carbon dioxide gas-desorptionperformance of these carbon dioxide gas absorbents. Since the gaseouscondition in this experiment was set to such that carbon dioxide gas wasabout 100 vol %, the carbon dioxide gas desorption reaction wasperformed at a temperature of about 800° C. However, the temperature ofthe carbon dioxide gas desorption reaction can be varied depending onthis gaseous condition.

[0062] Additionally, the melting point of the films formed on thesurfaces of these carbon dioxide gas absorbents was also measured.

[0063] Moreover, the following test was also performed to evaluate theserviceable life of these carbon dioxide gas absorbents.

[0064] First of all, the carbon dioxide gas absorbents obtained inExamples I-1 to I-6, and Comparative Examples I-1 and I-2 wererespectively housed inside the box type electric furnace. Then, thiselectric furnace was maintained at a temperature of about 500° C. forone hour while permitting a mixed gas comprising about 20 vol % ofcarbon dioxide gas and about 80 vol % of nitrogen gas to flowtherethrough, thereby enabling carbon dioxide gas to be absorbedtherein. Thereafter, these carbon dioxide gas absorbents were oncecooled to room temperature to measure any increment of the weightthereof. Then, under a gaseous condition where carbon dioxide gas wasset to about 100 vol %, the temperature of each of the carbon dioxidegas absorbents obtained in Examples and Comparative Examples wasmaintained at the melting point of each of the films formed on thesurfaces of these carbon dioxide gas absorbents to permit carbon dioxidegas to be released therefrom for two hours. This experiment on theabsorption and desorption of carbon dioxide gas in these carbon dioxidegas absorbents was repeated five times under the same conditions, andany increment of the weight thereof was respectively measured afterfinishing the fifth experiment. Then, the ratio of absorption of carbondioxide gas, i.e. (the absorption at the fifth experiment/the absorptionat the first experiment) was determined from the ratio of increment inweight of these carbon dioxide gas absorbents, i.e. (the absorption atthe fifth experiment/the absorption at the first experiment).

[0065] The results are shown in the following Table 2.

[0066] Incidentally, when the same experiment as describe above wasperformed by substituting nitrogen gas (without the inclusion of carbondioxide gas) for the aforementioned mixed gas, any increase or decreasein weight was not recognized in these carbon dioxide gas absorbents.TABLE 2 Time required for attaining Time required 130 wt % of forcompletely Melting Absorption initial weight releasing CO₂ point ofratio (min) (min) film (° C.) (5th/1st) Example I-1 45 30 765 0.95Example I-2 70 20 740 1   Example I-3 60 25 750 1   Example I-4 30 45785 0.9  Example I-5 25 60 800 0.8  Example I-6 60 40 780 0.85Comparative 20 — 850 0.25 Example I-1 Comparative 120  15 730 1  Example I-2

[0067] As shown in Table 2, the carbon dioxide gas absorbents accordingto Examples I-1 to I-6 were found to take a relatively short time forenabling carbon dioxide gas to be completely released therefrom.Whereas, it was impossible in the case of Comparative Example I-1 tocompletely release carbon dioxide gas therefrom at a temperature ofabout 800° C. Therefore, the carbon dioxide gas absorbents according toExamples I-1 to I-6 were confirmed as being excellent in carbon dioxidegas-desorption performance.

[0068] With respect to the melting point of the film formed, ExamplesI-1 to I-6 showed a relatively low temperature of about 800° C. or less.By contrast, the melting point of the film formed in Comparative ExampleI-1 was as high as about 850° C. When the carbon dioxide gas-desorptionreaction was performed at the melting point of the film, the ratio ofabsorption of carbon dioxide gas, i.e. (the absorption at the fifthexperiment/the absorption at the first experiment) was relatively large,thus indicating a negligible degradation of the absorbent.

[0069] Whereas, in the case of Comparative Example I-1, the quantity ofthe absorption of carbon dioxide gas at the fifth experiment was merelyabout 25% of that of the first experiment, thus indicating a substantialdegradation of the absorbent. Further, since the melting point of thefilm is as high as about 850° C., the absorbent was considered as beingdeteriorated after several repetitions of the carbon dioxide gasabsorption reaction.

[0070] In Comparative Example I-1, the content of potassium carbonate inthe lithium silicate was 5 mol %. In Examples I-1 to I-6, the content ofalkali carbonate in the lithium silicate was 4.9 mol % or less.Therefore, it will be clear that if it is desired to prolong theserviceable life of the carbon dioxide gas absorbent containing animpurity, the content of alkali carbonate in the lithium silicate shouldbe confined to 4.9 mol %.

[0071] Further, the carbon dioxide gas absorbents according to ExamplesI-1 to I-6 were found to take a remarkably short time for enablingcarbon dioxide gas to be absorbed therein as compared with that ofComparative Example I-2, thereby confirming an excellent carbon dioxidegas-absorbing performance of the absorbents according to Examples I-1 toI-6. The content of potassium carbonate in the lithium silicate ofComparative Example I-2 was only 0.3 mol %. Whereas, the content ofalkali carbonate in the lithium silicate according to Examples I-1 toI-6 was 0.5 mol % or more. Therefore, it will be clear that if it isdesired to enhance the carbon dioxide gas absorption performance of thecarbon dioxide gas absorbent where an impurity such as aluminum isincluded therein, the content of alkali carbonate in the lithiumsilicate should be at least about 0.5 mol %.

[0072] It will be seen from Tables 1 and 2 that it is possible to obtaina carbon dioxide gas absorbent which is better in serviceable life ifthe content of alkali carbonate in the lithium silicate is confined tothe range of about 1 mol % to about 3 mol %.

EXAMPLE II

[0073] Lithium orthosilicate was prepared by following the sameprocedures as described in Example I-1. Then, to this lithiumorthosilicate, alkali carbonates were added as shown in the followingTable 3 to obtain the carbon dioxide gas absorbents of Examples II-1 toII-6 and of Comparative Examples II-1 and II-2. TABLE 3 Alkalicarbonates Content Kinds (mol %) Example II-1 K₂CO₃ 2 Example II-2 K₂CO₃0.5 Example II-3 K₂CO₃ 1 Example II-4 K₂CO₃ 3 Example II-5 K₂CO₃ 4.9Example II-6 Na₂CO₃ 2 Comparative K₂CO₃ 5 Example II-1 Comparative K₂CO₃0.3 Example II-2

[0074] By using each of these carbon dioxide gas absorbents, theabsorption of carbon dioxide gas was performed in an air flow containing500 ppm of carbon dioxide gas and under the conditions of: 25° C. intemperature and 40% in humidity to evaluate the carbon dioxidegas-absorbing performance thereof. In this absorption of carbon dioxidegas at room temperature, an apparatus shown in FIG. 2 was employed.

[0075] In the apparatus shown in FIG. 2, a resin tube 30 was filled witha carbon dioxide gas absorbent 31, and both ends of the tube 30 weresealed with an absorbent cotton 32. Air where the content of CO₂ gastherein was measured in advance was introduced into the tube 30 from oneend thereof to perform the absorption of carbon dioxide gas. The airafter the absorption of carbon dioxide gas was released from the otherend of the tube 30 to measure the content of carbon dioxide gas includedtherein. For the measurement of the CO₂ gas content, a CO₂ densitometer34 was used.

[0076] Thereafter, these carbon dioxide gas absorbents having carbondioxide gas absorbed therein were respectively place in an electricfurnace and heated at a temperature of 800° C. under a gaseous conditionwhere carbon dioxide gas was set to about 100 vol % to perform thedesorption of carbon dioxide gas.

[0077] The carbon dioxide gas desorption performance was evaluated withrespect to the following points.

[0078] (1) The time taken until the entrapment ratio of carbon dioxidegas became 80%.

[0079] (2) The time taken until the carbon dioxide gas was completelyreleased.

[0080] (3) The melting point of the film formed on the surface of thecarbon dioxide gas absorbent.

[0081] (4) The time taken until the entrapment ratio of carbon dioxidegas became 80% after five repetitions of the carbon dioxide gasabsorption/desorption experiment.

[0082] Incidentally, the time until the entrapment ratio of carbondioxide gas became 80% was obtained by calculating the ratio between theCO₂ concentration at the outlet port and the CO₂ concentration at theinlet port and plotting the concentrations versus time.

[0083] The time taken until the carbon dioxide gas was completelyreleased was measured in the same manner as described above. The resultsare summarized in the following TABLE 4 Ratio of time until Time Timeentrapping required for required for ratio attaining 80% completelyMelting becomes 80% of entrapping releasing point of after 5 times ratio(min) CO₂ (min) film (° C.) of repetition Example II-1 60 25 765 0.9 Example II-2 30 17 740 0.95 Example II-3 45 22 750 0.95 Example II-4 9040 785 0.85 Example II-5 100  50 800 0.75 Example II-6 55 34 780 0.8 Comparative 120  — 850 0.1  Example II-1 Comparative 10 13 730 0.95Example II-2

[0084] As shown in Table 4, since the content of alkali carbonate in thelithium silicate according to Comparative Example II-1 was 5 mol %, atime of as long as 120 minutes was required until the entrapment ratioof carbon dioxide gas became 80%. However the melting point of the filmformed was as high as 850≦ C. Therefore, it was impossible to completelyrelease CO₂, thus resulting in the deterioration in repeatability. Inthe case of the carbon dioxide gas absorbent of Comparative ExampleII-2, since the content of alkali carbonate was 0.3 mol %, the timerequired until the entrapment ratio of carbon dioxide gas became 80% wasrelatively short and hence was defective in this respect.

[0085] Whereas, in the cases of carbon dioxide gas absorbents accordingto Examples II-1 to II-6 where the content of alkali carbonates in thelithium silicate was confined within the range of 0.5 mol % to 4.9 mol%, excellent results were obtained in all respects.

[0086] As explained above, it was confirmed that the carbon dioxide gasabsorbents where the content of alkali carbonates in the lithiumsilicate was confined within the range of 0.5 mol % to 4.9 mol % werecapable of excellent absorption of carbon dioxide gas even at roomtemperature, and that the serviceable life thereof was sufficientlylong.

[0087] In the foregoing examples, only embodiments where lithiumorthosilicate was employed as a lithium silicate, and potassiumcarbonate and sodium carbonate were employed as an alkali carbonate wereillustrated. However, other materials selected from any kinds of lithiumsilicate and alkali carbonate may be used to obtain almost the sameeffects as mentioned above.

[0088] As explained above, it is possible, according to one aspect ofthe present invention, to provide a carbon dioxide gas absorbent whichis sufficiently long in serviceable life. Further, it is also possible,according to another aspect of the present invention, to provide acarbon dioxide separating apparatus which is capable of effectivelyperforming the separation and recovery of carbon dioxide gas.

[0089] Additional advantages and modifications will readily occur tothose skilled in the art. Therefore, the invention is its broaderaspects is not limited to the specific details and representativeembodiments shown and described herein. Accordingly, variousmodifications may be made without departing from the spirit or scope ofthe general inventive concept as defined by the appended claims andtheir equivalents.

What is claimed is:
 1. A carbon dioxide gas absorbent comprising:lithium silicate; 0.5 mol % to 4.9 mol % of alkali carbonate per mole ofsaid lithium silicate; and at least one element selected from the groupconsisting of aluminum, magnesium, calcium, iron, titanium and carbon.2. The carbon dioxide gas absorbent according to claim 1, wherein thecontent of said alkali carbonate is in the range of 1 mol % to 3 mol %per mole of said lithium silicate.
 3. The carbon dioxide gas absorbentaccording to claim 1, wherein said lithium silicate comprises at leastone of compound selected from the group consisting of Li₄SiO₄, Li₂SiO₃,Li₆Si₂O₇ and Li₈SiO₆.
 4. The carbon dioxide gas absorbent according toclaim 1, wherein said lithium silicate comprises Li₄SiO₄.
 5. The carbondioxide gas absorbent according to claim 1, wherein said alkalicarbonate comprises at least one compound selected from the groupconsisting of potassium carbonate, sodium carbonate and lithiumcarbonate.
 6. The carbon dioxide gas absorbent according to claim 5,wherein said alkali carbonate comprises potassium carbonate.
 7. Thecarbon dioxide gas absorbent according to claim 1, which substantiallyassumes a powdery form.
 8. The carbon dioxide gas absorbent according toclaim 7, wherein said powdery formed one has an average particlediameter ranging from 0.5 μm to 50 μm.
 9. The carbon dioxide gasabsorbent according to claim 1, which substantially assumes a porousform.
 10. The carbon dioxide gas absorbent according to claim 9, whereinsaid porous formed one has a porosity ranging from 30% to 60%.
 11. Acarbon dioxide gas separating apparatus comprising: a reaction chamberhaving a carbon dioxide gas inlet and a product gas outlet; a carbondioxide gas absorbent placed in said reaction chamber; said carbondioxide gas absorbent comprising lithium silicate; 0.5 mol % to 4.9 mol% of alkali carbonate per mole of said lithium silicate; and at leastone element selected from the group consisting of aluminum, magnesium,calcium, iron, titanium and carbon; and a heater heating said reactionchamber and disposed around said reaction chamber.
 12. The carbondioxide gas separating apparatus according to claim 11, wherein thecontent of said alkali carbonate is in the range of 1 mol % to 3 mol %per mole of said lithium silicate.
 13. The carbon dioxide gas separatingapparatus according to claim 11, wherein said lithium silicate comprisesat least one of compound selected from the group consisting of Li₄SiO₄,Li₂SiO₃, Li₆Si₂O₇ and Li₈SiO₆.
 14. The carbon dioxide gas separatingapparatus according to claim 11, wherein said lithium silicate comprisesLi₄SiO₄.
 15. The carbon dioxide gas separating apparatus according toclaim 11, wherein said alkali carbonate comprises at least one compoundselected from the group consisting of potassium carbonate, sodiumcarbonate and lithium carbonate.
 16. The carbon dioxide gas separatingapparatus according to claim 15, wherein said alkali carbonate comprisespotassium carbonate.
 17. The carbon dioxide gas separating apparatusaccording to claim 11, wherein said carbon dioxide gas absorbentsubstantially assumes a powdery form.
 18. The carbon dioxide gasseparating apparatus according to claim 17, wherein said powdery formedone has an average particle diameter ranging from 0.5 μm to 50 μm. 19.The carbon dioxide gas separating apparatus according to claim 11,wherein said carbon dioxide gas absorbent substantially assumes a porousform.
 20. The carbon dioxide gas separating apparatus according to claim19, wherein said porous formed one has a porosity ranging from 30% to60%.